The semaphorins constitute a large family of evolutionally conserved glycoproteins that are defined by a characteristic semaphorin domain of approximately 500 amino acids (1-3). The first vertebrate semaphorin, collapsing in chick, was identified by its ability to induce growth cone collapse (4). Consistent with this function, its mammalian homologue, sema III, has been shown to repel specific subsets of sensory axons (5). As a result of these and other studies, Coll-1/sema III/D has been implicated in the patterning of sensory axon projections into the ventral spinal cord and cranial nerve projections into the periphery (6-11).
Several other semaphorins have also been implicated as repulsive and/or attractive cues in axon guidance, axon fasciculation, and synapse formation (1, 12-17). In addition, members of semaphorin family have been implicated in functions outside the nervous system, including bone skeleton and heart formation (9), immune function (18, 19), tumor suppression (20-22), and conferring drug resistance to cells (23).
Recent studies have identified the first semaphorin receptor as a member of the neuropilin family. Neuropilin-1 is a high affinity receptor for sema III, E and IV, whereas neuropilin-2 binds differentially to the subfamily of secreted semaphorins (24-27).
The vertebrate semaphorin family can be classified into several phylogenetically distinct subfamilies (15). Each subfamily has a unique structural arrangement of protein domains. The secreted members of the semaphorin family contain a characteristic semaphorin domain at the N-terminus, followed by an immunoglobulin (Ig) domain and a stretch of basic amino acids in the carboxyl-terminal region. Between the N-terminal semaphorin domain and the transmembrane spanning region, the transmembrane semaphorins contain several alternative structural motifs including either an Ig domain, a stretch of thrombospondin repeats, or a sequence with no obvious domain homology. Interestingly, semaphorin-like sequences have been identified in the genomes of poxviruses (1) and alcelaphine herpesvirus-1 (28), occupying unique branches of the semaphorin phylogenetic tree. Here we report the identification of a GPI-linked human semaphorin--semaphorin K1--which is homologous to the semaphorin encoded by alcelaphine herpesvirus-1 and show that semaphrin K1 polypeptides and nucleic acids are bioactive in modulating nervous and immune system function.
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